Back Pressure (BP)
Compiled by Chris Hugo, Powerflow-Briardene Durban North
Basic Exhaust Theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream but originate in pulses, the more pulses that are produced the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow.
The Myth of Backpressure
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what its consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a misguided notion.
How This Myth Came To Be?
I often wonder how the myth "engines needs backpressure" came to be. It is believed to be a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance someone goes to an exhaust shop and he decides he is going to upgrade his exhaust from a 48mm diameter to a 76mm diameter pipe. Once it's done the owner notices that he seems to have lost a good bit of power throughout the vehicles performance. He then makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power. Therefore the motor must need backpressure in order to make power?" what he did not realize is that he killed off all his flow velocity by using such a ridiculously big pipe. It would have being possible for "him" to achieve close to zero backpressure with much narrower pipe and in that way he wouldn't have lost all his flow velocity.
The Idea Behind Backpressure
An exhaust system's purpose is to evacuate gasses from the cylinder, and logic would say that a bigger pipe would evacuate more of those gasses. This "bigger is better" idea doesn't always ring true; however there can actually be power loss at lower rpm's when the exhaust diameter is increased. This is due to a decrease in exhaust velocity which leads to the presence of turbulence. This turbulence can reduce performance, and that leads people to believe that less back pressure causes a loss in torque.
To better Understand This You Should Understand What These Terms Mean
Backpressure: is basically the pressure that opposes the desired exhaust flow.
Exhaust velocity: is how fast the exhaust gas is moving through the pipe. (To an extent, a smaller diameter pipe will increase velocity, and a larger diameter pipe will decrease velocity). It is also dependant on the volume of the exhaust gases produced by the engine as well as low revs vs. high revs and stock standard exhaust vs. high performance exhaust.
Turbulence: is the inefficient tumbling of exhaust gas flow caused by lack of velocity.
Exhaust pulses: Yes that is very true an exhaust pulses and behind each pulse it creates lower pressure wave to help pull the gases quicker out of the combustion chamber.
Basically the optimum exhaust system balances between having a very low to non back pressure and high velocity while avoiding turbulence. It is good to reduce the restrictions on the flow, but if too much velocity is lost, you will lose performance as well.
What can cause Backpressure and Its Effect on Engines?
Exhaust components like silencers and manifolds and tight bends are a source of engine exhaust back pressure, increased backpressure can cause increased emissions, increase fuel consumption, and can have a negative effect on the engines performance. NOTE: When you have a good amount of exhaust gases travelling back into the combustion chamber it reduces the capacity of your combustion leading to loss of performance and torque, increase in fuel usage and rough idling. Refer to EGR on our technical page for more information.
Benefits of Having a Free Flow System.
A free flow exhaust system was design to eliminate backpressure.
A free-flowing exhaust system is usually seen as one in which the mufflers have been removed or converted to "straight-through" units that offer less resistance to the gasses passing through the exhaust pipe. In theory, the lower resistance would improve engine performance, which would be especially noticeable at high engine speeds.
Components That Causes Backpressure
- Stock Manifold.
- Stock Silencers.( Non Free Flow)
- Diesel Particle Filters & Catalytic Converters
- Poorly shaped bends with a too small radius
- Incorrectly sized tubing
What Can We Do About It?
You can replace all stock items with specialised and engineered aftermarket branches and silencer that will definitely boost your performance and fuel economics.
Stock manifolds & Aftermarket Branches
Exhaust manifolds traditionally funnel spent exhaust gasses and causes restrictions from the engine. They are designed to be practical to get your exhaust gases from point A to point B; these heavy chunks of cast iron do their job adequately but don't inspire performance. Horsepower and torque is sacrificed in the name of lower manufacturing costs and increased emissions, zapping acceleration from your ride. Even worse, some factory manifolds are prone to cracking, putting even the toughest vehicles out of commission.
As you can see in the picture above you exhaust gases are always conflicting each other trying to exit the exhaust manifold as quick as possible, but the cross flow and the small outlet restrict the flow of the exhaust causing back pressure and reduce performance.
Aftermarket Branches are one of the easiest bolt-on modifications you can make to your vehicle to improve your engine's performance. Branches make it easier for your engine to push exhaust gases out of the cylinders. Your vehicle's engine produces all of its power during the power stroke. During this stroke, the fuel in the cylinder burns and expands to generate power. Once the exhaust gases vacate the cylinder, they end up in the branches and then flow into one pipe toward the collector. The exhaust branches help eliminates manifold's back pressure. So, instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe, and then they come together in a larger pipe called the collector. Powerflow has a wide range of performance branches that is specialized for each individual vehicle.
Stock Silencer & Free Flow Silencers
Stock silencers are designed to reduce the sound level of a vehicle significantly, to achieve these noise reduction they build the silencer with chambers to restrict the exhaust gases temporarily in the silencer itself so that the sound waves stays in the fibres of the silencer longer before exiting through the exhaust pipes.
Free Flow Silencer:
Free flow exhaust silencer has no restrictions and was designed to give your vehicle better exhaust flow and maintain the velocity of the flow throughout the entire system. With this design your vehicle will most definitely benefit from it giving you the power and fuel efficiency you always wanted. Powerflow are the pioneers of the free flow silencer, we have engineered and tested free flow silencers for many years.
A performance exhaust shop will have specialised pipe bends to remove all restrictions, they are called mandrel bends these bends are designed to have an equal flow from the start of a bend to the end of the bend to help the exhaust gases flow without any restrictions. Pressure Bent Tubes are squashed bends, where the pipe has being squashed is where the resistance comes into play affecting the velocity of the gases flow.
Diesel Particle Filters & Catalytic Converters
These eco friendly monsters have plenty environmental benefits, but are one of the worst and restrictive designs fitted on today's vehicles. They have a minute honey comb design that likes clogging up with soot. You can read more about CAT'S and DPF on our technical page.
A world renowned exhaust developer has done extensive tests to find the optimum tube diameter at different sections of a exhaust and every variable was Dyno tested and without divulging any specific detail the broad conclusion was:
"A wide range of testing was performed on a range of different diameters and lengths of exhaust pipes in varying locations of the exhaust system. All of these tests demonstrated different ways of changing exhaust pulse scavenging and backpressure. In all cases the results show scavenging is beneficial and back pressure is detrimental to engine performance. Engine performance can be maximized by utilizing a continually expanding exhaust system design with the smallest (optimally sized) pipes closest to the engine and progressively expanding to larger and larger diameter pipes as the flow moves away from the engine."
Our technical advisor would gladly assist with any guidance and advice you may require in upgrading your vehicles exhaust system.